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Why the flat sheet looks wrong
Fold a sheet in half three times and you have 16 pages. Number them, unfold the sheet, and the numbers are scattered: on one side 1, 4, 5, 8, 9, 12, 13 and 16, on the other 2, 3, 6, 7, 10, 11, 14 and 15, half of them upside down. Pages that sit side by side on the sheet add up to 17 — one more than the signature has.
That folded and numbered blank sheet is the folding dummy, and it is still the surest way to check an imposition before it goes to plate.
The binding decides the page order
Binding is not a label added at the end; it decides which page goes into which position.
- Saddle stitch — signatures are nested one inside the other and stapled through the spine. The outermost signature carries the first and the last pages of the booklet together. The page count must be a multiple of four.
- Perfect binding — signatures are gathered one on top of the other, the spine is milled off and glued. Each signature is a consecutive block of pages. Milling takes about 3 mm from the spine, so the inner margin has to allow for it.
- Section sewing — gathered like perfect binding, but each signature is sewn through its fold; nothing is milled away.
Creep
In a saddle-stitched booklet the inner sheets wrap around fewer sheets than the outer ones, so after folding they stick out further at the fore-edge. When the booklet is trimmed, the inner pages lose more than the outer ones and text near the edge can be cut off.
Creep allowance corrects this by shifting the pages of the inner signatures slightly toward the spine. The amount grows with the paper's thickness and the number of nested sheets. Perfect-bound books are gathered, not nested, so they have no creep.
Splitting a book into signatures
A 48-page book is three signatures of 16. A 40-page book does not divide evenly: it becomes two 16s and an 8, or a 32 and an 8. The small leftover signature costs a separate run, plates and make-ready, so the page count is often adjusted with the publisher before the job is set.
How many pages fit on one signature depends on the sheet: a sheet that takes a 32-page signature halves the number of signatures — and the plates — compared with one that takes 16.
Turning the sheet
A signature printed sheetwise has separate plates for front and back. In work-and-turn the front and back of the signature share one plate and each sheet yields two signatures — which also means creep cannot vary between them. In work-and-tumble the sheet turns head to tail, the gripper moves to the other edge, and the colour bar cannot print where the second pass is gripped.
Page numbers
If the file comes without page numbers, the imposition prints them. Each number has to turn with its own page: half of the pages on a signature are upside down, and a number set to the sheet instead of the page would print head-down through half the book.